The Ignition of an internal combustion engine is comprised of two basic types: spark ignition and compression ignition.
Spark Ignition, Primarily Gasoline Engines
The energy emission from a spark in a gasoline engine varies widely depending on a large number of factors related to materials and design. Three primary groups can be identified. A) The older style rotary distributor with either a coil transformer or capacitor discharge through the distributor and wires leading to the spark plugs. B) The slightly newer master coil pack which is controlled by an ECU (engine control unit) and a wire leading to each plug. C) The most modern is individual coil packs, where each ignition coil is mounted directly onto each spark plug, where the current/power source for the plug is separate from the ignition pulse and controlled by an ECU.
Historically, to read the revolutions per minute (RPM) from an engine one would attach a small transformer around one spark plug wire. The resulting pulse would then changed to a logic pulse by some external circuitry, after which a controller or computer would measure the frequency of the pulses and determine RPM (revolutions per minute). In the case of a master coil pack, often the spark plug wires are much heavier and often do not provide enough signal for the sensing transformer to work properly. In the case of individual coil packs, there are no spark plug wires. To measure RPM a different type of probe is used, typically a low voltage version of the standard transformer probe. To use this probe, the wires leading to one plug must be freed from its protective wrapping and only the pulse line placed inside the probe “clamp”. An example of a system that works with all 3 of these types is the SmartTach from Accurate Technologies Inc. This “classic” approach leads to several significant issues. Often the spark plug wires are difficult to access, especially in vehicles imported from over-seas. Also, with individual coil packs, it requires the ignition wire be modified by either cutting or compressing the protecting “loom” so that the wire can be accessible enough to attach the probe. Further, in many cars that contain a single master coil, the coil is located in a place which cannot be reached without disassembling the car. In many cases heavy duty, low EMI plug wires are used that attenuate the signal to such a low level that it cannot be seen using conventional equipment. Also, there is no “standard” location or wiring path as these vary significantly from vehicle to vehicle.
As such, what is needed is a sensor that is capable of such significant sensitivity that it can detect levels as low as 2 microvolts to overcome these issues.
Compression Ignition, Primarily Diesel Engines
Diesel engines do not use spark plugs. Diesel fuel ignites when under very high pressure and high temperatures. A diesel engine has glow plugs, which are preheated when the engine is started and continue to remain hot without any additional power. As the fuel is compressed, it ignites. Historically to read RPM from a diesel engine one of three methods are used resulting in various issues. A) Magnetic, a magnet is attached to the rotating mass, a small fixed hall-effect sensor is used to read the magnet as it passes by the sensor. This requires physical mounting inside the engine compartment. B) Optical, a small optical reflector (tape, paint, etc.) is placed on the rotating mass. An optical emitter/detector is placed in a fixed location so that it can sense the passing of the reflector. C) Fuel pulse, a highly sensitive pressure transducer is placed on the fuel line. This transducer senses the changes in fuel pressure as the injectors are fired. In all of these cases, a device must be attached to the vehicle such that it will not fall off or become misaligned during testing.
As such, what is needed is a device that has enough sensitivity to be able to see the small signals present when an injector solenoid is fired to overcome these issues.